Hard disk drives are computer-readable storage media that can be used by computing devices to store information for later use and retrieval. FIG. 1 illustrates an example of a hard disk drive 100. The hard disk drive 100 stores information on one or more circularly-shaped platters 102, which include a material that can maintain a magnetic state. Information is written to a portion of a platter 102 by changing a magnetic state of the portion and information can be read from the platter 102 by detecting a magnetic state of a portion.
The hard disk drive 100 includes one or more devices 106 to read information from and write information to a platter 102. The devices 106, which are called magnetic heads herein, are may be mounted on one end of a structure 104 that extends over the platters 102. This structure 104 on which the magnetic heads 106 are mounted is referred to herein as an actuator arm. When the hard disk drive 100 is operating, the actuator arm 104 is driven by a motor (referred to herein as an actuator) 108 such that the end of the arm 104 (and thus the magnetic heads 106) moves toward and away from a center of the circularly-shaped platters 102. During operation of the hard disk drive 100, the platters 102 are driven to rotate by a motor 110, which is referred to herein as a spindle motor 110. By rotating the platters 102 and moving the magnetic head(s) 106 toward a center of the platters 102 or toward an edge of the platters 102, the magnetic heads 106 may read information from and/or write information to different portions of the platters 102.
The platters 102 and the magnetic heads 106 are each made of materials that may be easily damaged if contacted. As a result, the actuator arm 104 is structured to keep the magnetic heads 106 suspended close to, but not touching, the platters 102. During normal operation of the hard disk drive 100, the spindle motor 110 drives the platters 102 to rotate relatively quickly. Some contemporary hard disk drives, for example, rotate platters at 7600 rotations-per-minute (rpm). This movement of the platters 102 causes a movement of air on the surface of the platters 102. The actuator arm 104 has a shape that enables the actuator arm 104 to be suspended over the platters 102 by this movement of air, such that the actuator arm 104 does not need to support itself over the platters 102 and does not need additional support structures.
From time to time, the hard disk drive 100 receives an instruction from a computing device to stop rotating the platters 102. The computing device may transmit the instruction when the computing device is undergoing a power transition, such as when the compute device is to be placed into an off state or a power-saving state. When the platters 102 stop moving, the movement of air will also stop and thus will no longer suspend the actuator arm 104 over the platters 102. If the actuator arm 104 is over the platters 102 when the platters 102 stop moving, the actuator arm 104 could fall onto the platters, resulting in damage to the platters 102 and/or to the magnetic heads 104. Accordingly, when the hard disk drive 100 receives an instruction to stop rotating the platters 102, a control circuit 112 included in the hard disk drive 100 will, in response to the instruction, control the actuator 108 to drive the actuator arm 104 away from the platters 102 and to a resting position 114. The actuator 108 drives the actuator arm 104 to the resting position 114 before the platters 102 stop moving, which means that the movement of air suspends the arm 104 above the platters 102 while the arm 104 is being moved to the resting position 114 and that the heads 106 will not contact and damage the platters 102.
Therefore, under normal operation, the hard disk drive 100 receives an instruction prior to stoppage of the rotation of the platters 102 and, in response to the instruction, the actuator arm 104 is moved to a resting position 114 away from the platters 102.
In the case of a sudden loss of external power to the hard disk drive 100, however, the spindle motor 110 will not be powered and will not be able to drive rotation of the platters 102. In this case, the platters 102 will stop moving, but the hard disk drive 100 will often not have received an instruction to stop the rotation of the platters 102. If the actuator arm 104 is above the platters 102 when external power is lost, the actuator arm 104 must be moved away from the platters 102, to the resting position 114, before damage results. If the platters 102 are rotating at the time that external power is lost, due to inertia the platters 102 may continue to rotate for a short time period, which will create a movement of air for the short time period. If the actuator arm 104 is moved to the resting position 114 before the end of the short time period, the actuator arm 104 will be suspended by the movement of air and there would be no damaging contact between the magnetic heads 106 and platters 102.
Of course, in the event of a loss of external power, there is no external power available for driving the control circuit 112 or the actuator 108 during the short time period. Several techniques have therefore been proposed for providing power to the control circuit 112 and actuator 108 during the short time period.
FIG. 2 illustrates a first example of a way to provide power to an actuator and a control circuit. FIG. 2 illustrates a circuit 200 for driving an actuator arm in the event of a loss of external power to a hard disk. In the example of FIG. 2, the circuit 200 includes a capacitor 202 that is charged during operation of the hard disk and, following a power loss, the capacitor 202 provides power to the control circuit 204 and to the actuator 206. The capacitor 202 provides all power to the control circuit 204 and to the actuator 206 during the time period between the loss of external power and the actuator arm reaching the resting position.
FIG. 3 illustrates a second example of a way to provide power to an actuator and a control circuit. In the circuit 300 of FIG. 3, the spindle motor 308 (illustrated as a DC equivalent of a spindle motor) of the circuit is operated as a power source for the circuit 300. The spindle motor 308 drives rotation of the platters of the hard disk during normal operation of the hard disk drive. Following a loss of external power, the platters and the spindle motor 308 will continue to rotate due to inertia. The rotating of the motor 308 due to the inertia will create electrical power due to the “back electromotive force,” designated as Vbemf in FIG. 3. This power can be drawn from the motor 308 and used to power the circuit 300. Some of the power created by the spindle motor 308 will be consumed by the motor 308 and electrical characteristics of the motor 308, which are modeled in FIG. 3 as the components 310A, 310B. Remaining power can be drawn from the motor 208 and used to power the control circuit 304 and the actuator 306 during the short period.
FIG. 4 illustrates a third example of a way to provide power to an actuator and a control circuit. FIG. 4 illustrates a circuit 400 that can be considered a blend of the circuits illustrated in FIGS. 2 and 3. The circuit 400 includes a capacitor 402 that is charged during operation of the hard disk and, following a power loss, provides power to the control circuit 404. The circuit 400 also includes a spindle motor 408 (illustrated as a DC equivalent of the spindle motor 408) from which power can be drawn following a loss of external power, as discussed above in connection with FIG. 3. Some of the power drawn from the spindle motor 408 may be consumed by the motor 408 according to electrical characteristics of the motor 408, as modeled by components 410A, 410B illustrated in FIG. 4. The power drawn from the motor 408 is used to power the actuator 406.